This invention relates to error correcting circuitry and more particularly to digital error correcting circuitry for use in servo control systems and which may be implemented inexpensively for use in industrial and commercial applications.
High accuracy servo control systems have been used for many years in a variety of military and aerospace applications to position movable control members. Such applications include the positioning of antennas and optical sensors. A key element in these control systems is the position transducer used to determine the position of the movable control member, since the overall performance of the control system is a function of the accuracy, repeatability and linearity of the position transducer. Historically, many types of transducers have been employed to measure position such as potentiometers, resolvers, differential transformers, magnetic encoders, optical encoders and the like.
Several of these types of transducers provide position determining signals having a periodic waveform. These signals may be in the form of sine or cosine waves, where the amplitude, slope and polarity of the waveforms may be used to determine the position of the movable control member. One such transducer of this type is the incremental position optical encoder which includes a light source and a group of photo sensors. Placed between the light source and the photo sensors are a fixed reticle and a movable disk which is mounted to a rotatable control member. The disk and reticle are each provided with a pattern of clear apertures and opaque areas. As the movable disk rotates with the control member, light paths from the light source to the photo sensors are created by the juxtaposition of the apertures in the disk and the reticle. The light paths are detected by the photo sensors which are positioned with respect to the disks to create two output signals in response to the rotation of the moving disk. These two signals are typically in the form of sine or cosine waves displaced in phase ninety degrees with respect to each other. Each of the sine and cosine waves represent the incremental rotation of the movable disk by a distance equal to the spacings between apertures.
Counting the number of sine or square waves that occur as the control member rotates provides a determination of the position of the member. Prior art techniques have also been developed for counting the number of zero crossings which occur in the encoder output signals. These techniques permit resolving the position of the motor shaft into four parts for each of the apertures in the disk pattern. Techniques have also been developed to measure the slopes of the encoder signal waveforms to provide increased resolution of the shaft position.
In practice, the encoder output signals include errors in both offset and amplitude due to mechanical and electrical tolerances of the components used in the encoder construction. To correct for these errors, prior art control systems provide a variety of manually adjustable circuit elements, usually in the form of potentiometers. These elements must be individually adjusted by trained personnel using electronic test equipment to verify the adjustments. It is the nature of the signal errors that they may vary as a function of time and temperature due to component tolerance drift. Accordingly, even though the control system circuit is initially adjusted to correct for signal errors, such adjustments must be repeated at regular intervals during the use of the control system to maintain control accuracy. These adjustments are costly to perform and are dependent upon the skill of the service personnel. Further, there is no way to compensate for variations in signal errors which occur between intervals of adjustment.
Accordingly, it is an object of the present invention to provide new and improved error-correcting circuitry for use in servo control systems.
It is another object of the present invention to provide digital error-correcting circuitry which automatically compensates for signal errors without requiring manual adjustments.
It is still another object of the present invention to provide digital error-correcting circuitry which automatically revises the signal error compensation to account for error variations.
It is still another object of the present invention to provide digital error-correcting circuitry which automatically compensates for both offset errors and amplitude errors.